The invention relates to an electric drive device with a DC motor, in particular with a permanent-magnetically excited air core coil motor, with a control circuit comprising an electronic commutator.
Electronically commutated, permanent-magnetically excited small DC motors of low inductance, such as air core coil motors, satisfy high requirements as regards quiet running. This property becomes increasingly important for spindle drives in computer drive units, because the achievable storage densities increase. Air core coil motors are particularly suitable here, because they generate no interfering detent torques and radial forces.
Known spindle drives are fitted exclusively with slotted-core iron armature motors. There are suggestions for hard disk drives with air core coil motors (foil motors) with internal rotor as described, for example, in U.S. Pat. No. 5,714,828. A new development in the field of electronic commutation of motors for hard disk drives is xe2x80x9cphase current shapingxe2x80x9d. Here the phase current is modulated in the three individual phases with the object of achieving a certainxe2x80x94for example approximately sinusoidalxe2x80x94gradient of the phase currents. Usually the motor voltage is pulse width modulated (PWM) in this case, because otherwise the switching losses would be very high. The PWM duty cycle serves as the manipulated variable. This method as a rule requires a PLL-supported commutation control on account of the more difficult zero passage recognition of the induced voltage. Furthermore, such a phase current shaping cannot be transferred to air core coil motors (for example with foil windings) without modifications. These motors indeed have considerably smaller electric time constants compared with the iron armature types (i.e. approximately one tenth) and require a correspondingly higher pulse frequency of the PWM, which in its turn can be realized with additional expenditure only.
EP 0773624 discloses another method of reducing the torque ripple through modification of the current in the intermediate circuit only, while retaining the conventional 120xc2x0 square wave commutation, preferably by sensorless commutation (EMF commutation). Here the motor voltage is raised after each commutation moment in order to achieve a somewhat more even intermediate circuit current and in addition to avoid torque glitches after switching caused by the operation and the inductance. The signal for this is obtained from the discharging process of an RC member. The torque ripple can indeed be reduced in this manner, but only to a certain degree, because it is to be observed that the product of current and flux linking change is to be kept constant at all times. This, however, requires a continuous adaptation of the current. This method is not very suitable for motors with a very small electric time constant.
It is an object of the invention to reduce commutation-dependent torque fluctuations of permanent-magnetically excited motors with air core coils.
According to the invention, this object is achieved in that a derivation of a control signal is obtained from an induced motor voltage detected by a measuring device and from a reference value which serves to regulate the speed of the DC motor, and in that the derived control signal serves to achieve a substantially constant torque of the DC motor through adjustment of the motor currents (ia, ib, ic). This motor control reduces the commutation-dependent torque ripple of low-inductance small DC motors. For this purpose, the 120xc2x0 square wave commutation with zero passage detection is retained, and the intermediate circuit current only is modulated, so that a modulation of the single phase currents is omitted and a conventional commutation method, such as sensorless commutation (EMF commutation) can be used. The signal necessary for this is obtained in a simple manner from the induced voltage. The circuitry expenditure is clearly reduced in comparison with other methods thereby.
The embodiment as claimed in claim 2 renders possible a simple determination of the reference value for the motor current at which the motor torque is constant. The embodiment of claim 3 renders possible a determination of the reference value for the motor current at which the motor torque is constant, also if the motor speed is not constant.
The embodiment of claim 4 renders possible a simple adjustment of the intermediate circuit current by means of a longitudinal control and a control member, using the control signal.
In the embodiment of claim 5, the control member may be omitted in that an inverter belonging to the commutator is directly controlled by the control unit.